RNA plays an important role in cellular regulation -- either by its presence in active form or by its total absence. It ha been known for several years that transcription of DNA does not necessarily lead to productive, mature RNA molecules. Cleavage of these RNA molecules often is required for the RNA molecules to mature or to act as an intermediate in other processes (e.g., priming of DNA replication). These cleavage events are a subset of a general maturation pathway known as RNA processing. Work of this Intramural Research Project is concerned with two types of RNA processing, generation of RNA primers for DNA replication and ribosomal RNA processing in higher eukaryotes. We have studied the relationship between amino acid sequence and enzymatic activity of ribonuclease H (RNase H) by examination of mutants of Escherichia coli with altered RNase H activities, by determination of the amino acid sequence of Salmonella typhimurium RNase H and a yeast RNase H. The gene coding for the yeast RNase H has been located on the yeast genetic map to the right arm of chromosome XIII about 250 kilobase from the telomere. Inactivation of the yeast RNase H gene produces strains that exhibit normal growth phenotypes. The processing of ribosomal RNa in the mutant BHK cell line has also been studied. The temperature sensitive phenotype has been corrected by transfection of the mutanmt cells with a cDNA library from mouse.